Transformer coupled push-pull power converters, which use pulse-width modulation for output regulation, are well known. In such converters, two transistors are alternately switched between saturation and cut-off to supply pulsating d-c current to an output power transformer. The required d-c output voltage is obtained by rectifying and filtering the voltage appearing across the secondary of the transformer. Regulation of the output voltage is obtained by varying the pulse width of the drive signals to the power transistors under control of the output voltage.
To protect the power converters as well as the associated load equipment against possible damage due to short circuits and overloads, an over-current protection circuit is included. When the power converter is used to power communications equipment, two types of protection may be used, fused or unfused. The supply voltage for low voltage logic circuitry (nominally 5 volts) used in such communications equipment, is usually distributed without fusing to avoid the additional voltage drop across the fuse. Since the load is usually constant, there is no problem with overload. However, short circuits may develop in components or wiring used in the equipment. If a converter supplies current in the order of 20 amps or more, an over-current control may be used to avoid burning out portions of the equipment such as the printed circuit boards.
A reliable and simple way to obtain pulse-width modulation control in a d-c to d-c converter is by means of a magnetic amplifier. With such an arrangement, a saturable-core type oscillator generates square-wave pulses at a nominal frequency of say 20 KHz, which are coupled through a transformer to each base of the two power transistors. Saturation of the magnetic amplifier gate windings coupled to the transformer output, results in an effective short-circuit of the drive signal during a portion of each cycle, so that pulse-width modulation control can be obtained by controlling the firing angle of the magnetic amplifier in response to the output voltage or current.
In the past, attempts to regulate both the output voltage and provide over-current protection control through a single control winding in the magnetic amplifier have led to instability and control problems, since both control signals are generally attempting to function in opposite directions. For example, when a short-circuit occurs, the increased load will tend to decrease the output voltage so that the control circuit will respond by attempting to increase the pulse-width of the drive signals to the transistors. However, this will further increase the short-circuit current so that the over-current protection control will attempt to decrease the pulse-width. This may result in hunting or instability in the control signal applied to the control winding of the magnetic amplifier.